1. Field of the Invention
The invention is based on a method of manufacturing a gear wheel.
2. Discussion of Background
In the case of large gear wheels, special material selection and hardening are imperative in order to obtain the requisite properties of the tooth system of the gear wheels with regard to wear resistance and bending fatigue strength. The main manufacturing steps for a gear wheel consist of the forging of the blank, premachining to a predetermined size and material testing, hardening, annealing and finish machining to final size by means of grinding. A martensitic transformation is produced in the surface layer of the gear wheel by the hardening and annealing step and results in compressive residual stresses on the surface and tensile residual stresses in the interior of the wheel. With increasing ratio of length to diameter of the gear wheel, these internal stresses lead increasingly to three-dimensional tensile stresses, which at material defects may lead to stress peaks and spontaneous crack growth.
The stresses of operation, on the one hand as a result of temperature changes and on the other hand as a result of the centrifugal force, are superimposed on the residual stresses and additionally aggravate the situation by virtue of the fact that they can cause cyclic or unstable crack growth. Since the centrifugal-force stresses increase with the speed of the tooth system and the thermal stresses increase with the size, the risk of such problems becomes all the greater, the larger the gear wheel becomes and the more pronounced the crowning of the gear wheel becomes and the faster it rotates.